Conventional processing of reversal films and papers comprises a reversal step between the black and white development and the color development. An example of such processing is the Ektachrome E-6.RTM. process described in detail in Chimie et Physique Photographiques Tome 2, P Glafkides, 5th edition, pages 954 et seq.
During the reversal step, the tin is present initially in the form of a stannous salt in the reversal bath. A certain quantity of this bath is conveyed mechanically with the film into the color development bath by capillary effect at the perforations and by impregnation of the layer of gelatin on the films. The tin is principally in a chelated form in the color development bath.
In practice, the color development takes place in a tank in which the film being processed passes. New development bath is regularly added to compensate for the exhaustion of the substances. It is desirable to recycle the bath for environmental protection reasons. During recycling, the tin content increases. If it exceeds approximately 0.2 g/l, the sensitometric properties of the film are effected thereby. This is why it is desirable to extract, at least partially, the tin contained in the color development bath, without modifying the other components of the bath.
Recovery of the tin by electrolysis is described in many patents. For example UK patent 2 159 139 describes a process for recovering the tin from products or concentrates containing quantities of tin of around 10 to 30%. For this purpose, the starting material is subjected to decomposition by potash in the presence of an inert gas or air, and then washed with water at a high temperature, and the solution obtained is electrolysed with a current density of between 50 and 500 A/m.sup.2. In this way up to 90% of the tin in a solution containing 30 g/l is recovered by electrolysis.
The patent DD 119 441 describes a process for eliminating tin from solutions originating from the refining of tin by alkali electrolysis. To do this, the Na.sub.2 S electrolyte enriched with tin and carbonate is subjected continuously to a second electrolysis with a non-oxidising anode using DC at an anode current density greater 850 A/m.sup.2, a cathode current density below 100 A/m.sup.2 and a temperature below 30.degree. C. The electrolyte is then recycled. In this way the concentration of tin is reduced from 50-70 g/l to 1-3 g/l.
The patent DE 2 647 006 describes the recovery of tin from the scrap from tin-enamelled sheet metal in which, after heating the scrap, treating it with soda and washing it, the solution containing 30 to 80 g/l of tin is subjected to a two-stage electrolysis with current densities of 100-150 A/m.sup.2 in the first stage and 50-70 A/m.sup.2 in the second stage at 80.degree.-90.degree. C. In this way 30 to 90% of the tin is recovered.
The patent WO 9001077 describes a process of electrolytic precipitation for the recovery of metals contained in photographic processing solutions. In this process, the solution, which contains metal in the form of thiosulphate or sulphite is decomposed by electrolysis without there being any deposition of metal at the cathode. The metal is recovered in the form of metallic sulphur by filtration. This method makes it possible to recover various metals including Sn but it is preferably applied to the recovery of silver.
These patents of the prior art do not make it possible to recover, by electrolysis, the tin contained in organic solutions in which the tin is mainly in a chelated form.
U.S. Pat. No. 4,437,949 describes a process and a device for recovering tin or forming organic tin compounds by the electrolysis of halogenated organic complexes of tin obtained during the manufacture of organo-tin halides by the reaction of organic halides with tin. The solutions subjected to electrolysis comprise from 10 to 20 g/l of tin.
In this device, an anode is placed in an aqueous anolyte and the cathode in the catholyte, which forms a phase which is non-miscible with the water containing the halogenated tin complex. The electrical current is conveyed electrolytically between these two phases. The anolyte is separated from the catholyte by the liquid/liquid interface between the aqueous anolyte and the catholyte which is non-miscible with the aqueous anolyte. During the electrolysis, the organic phase is enriched with dendritic tin.